It has long been known to produce cellular concrete based on Portland cement, silica sand, and lime by formation of hydrogen gas therein caused by the reaction of aluminum powder with a suitable reactant. The products obtained by this method have low mechanical properties, from 10 to 15 bars under compression at 28 days, measured according to French standard NF P 15451, for a specific gravity of 0.50, and under normal conditions of hardening in the air, suffer a shrinkage which may be in excess of 5 mm/m. This shrinkage which extends over a year is the cause of cracking and powdering which may result in destruction of the material. Therefore, it is necessary to store products for a minimum period of 70 days, which necessitates considerable storage areas. If it is desired to avoid shrinkage and enhance mechanical properties, it is necessary to resort, after partial hardening, to autoclave treatment lasting 12 to 24 hours at a temperature of 170.degree. to 200.degree. C. at a pressure of 8 to 10 atm. This treatment is particularly burdensome in terms of investment and power requirement.
It is also known to produce composite elements comprising a light-weight central layer of alveolar structure made from board or of a cellular structure made from polymers and sandwiched between two facing layers made from plaster. But said elements present the disadvantage of having to be assembled by gluing the different layers together. Also their mechanical properties are poor, which prevents them being used as load bearers such as in building construction.
It is also known to produce, by molding, composite plaster tiles comprising an aleveolar central layer sandwiched between two facing layers. But said tiles exhibit the disadvantage of having insufficient mechanical strength to enable them to be assembled into load-bearing constructions.
It is further known to produce composite elements comprising a cellular central layer made from plaster and sandwiched between two non-cellular facing layers likewise made from plaster. Such elements can be produced by a method which consists in forming the first facing layer by pouring a plaster paste onto a mold bottom, then forming the cellular central layer by pouring onto the first layer, before the latter is set, a layer of plaster having a porogenic system incorporated therein, then in forming the second facing layer by pouring a plaster layer onto the cellular central layer before the latter has finished setting. However, by this method the making of the central cellular plaster layer is difficult because the porogenic systems used, based on hydrogen peroxide (oxygen generators) or on mixtures of aluminum sulphate and calcium carbonate (carbon dioxide generators), lead to an immediate evolution of gas from the time when the paste is mixed. Thus, not only is a major part of the porogenic system dissipated as a pure loss during the mixing and transfer of the paste from the mixing station into the mold, but it is difficult to obtain a cellular central layer having a properly reproducible density and thickness due to the gas losses which are of a random character. Moreover, these elements exhibit the disadvantage of having insufficient mechanical strength to enable them to be used in load-bearing constructions.